A conventional air conditioner for a vehicle includes an evaporator that cools and dehumidifies ventilation air to be blown into a vehicle interior by exchanging heat between a low-pressure side refrigerant in a refrigeration cycle and the ventilation air within the vehicle interior. When intended to suppress fog on a window glass using an inside-air circulation mode of air-conditioning by circulating the inside air without introducing the outside air into the vehicle interior, a compressor in the refrigeration cycle is operated, causing the evaporator to dehumidify the ventilation air in the vehicle interior.
Thus, the compressor consumes power in suppressing the fog on the window glass. Specifically, when the compressor is an electric compressor, its operation consumes electric power. In contrast, when the compressor is an engine-driven compressor, its operation leads to a deterioration of fuel efficiency, a reduction of output from the engine, and degradation in engine feeling.
In the related art, Patent Document 1 describes an air conditioner for a vehicle that can ensure the antifogging properties and fuel-saving properties by improving the determination accuracy when the window glass actually becomes fogged. With this structure, the frequency of operating the compressor to suppress fog on the window can be decreased, thereby reducing the power consumed for antifogging of the window glass.